1. Field of the Technology
The present application relates generally to read sensors of magnetic heads in data storage devices, and more particularly to read sensors of the current-perpendicular-to-plane (CPP) type.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads which include read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, may be used to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage. Within the general category of MR sensors is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g. nickel-iron, cobalt-iron, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g. copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
One of the ferromagnetic (FM) layers referred to as the pinned layer has its magnetization typically pinned by exchange coupling with an antiferromagnetic (AFM) layer (e.g., nickel-oxide, iron-manganese, or platinum-manganese). The pinning field generated by the AFM pinning layer should be greater than demagnetizing fields to ensure that the magnetization direction of the pinned layer remains fixed during application of external fields (e.g. fields from bits recorded on the disk). The magnetization of the other FM layer referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). The pinned layer may be part of an antiparallel (AP) pinned structure which includes an antiparallel coupling (APC) layer formed between first and second AP pinned layers. The first AP pinned layer, for example, may be the layer that is exchange coupled to and pinned by the AFM pinning layer. By strong antiparallel coupling between the first and second AP pinned layers, the magnetic moment of the second AP pinned layer is made antiparallel to the magnetic moment of the first AP pinned layer.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or AP pinned depending upon whether the pinned structure is one or more FM layers with a unidirectional magnetic moment or a pair of AP pinned layers separated by the APC layer with magnetic moments of the AP pinned layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located there between.
A read sensor may also be of a current-perpendicular-to-planes (CPP) type in which current flows perpendicular to the major planes of the sensor layers. First and second shield layers engage the bottom and the top, respectively, of the sensor so as to simultaneously serve as electrically conductive leads for the sensor. The CPP sensor may be contrasted with a current-in-plane (CIP) type sensor in which the current is conducted in planes parallel to the major thin film planes of the sensor. In a CPP sensor, when the spacer layer between the free layer and the AP pinned structure is nonmagnetic and electrically conductive (such as copper), the current is referred to as a “sense current”; however when the spacer layer is nonmagnetic and electrically nonconductive (such as aluminum oxide), the current is referred to as a “tunneling current”. Hereinafter, the current is referred to as a perpendicular current Ip which can be either a sense current or a tunneling current.
All conventional metallic CPP read sensors have several shortcomings. First, their resistance-area (RA) products are quite low. For typical sensor areas, this results in read sensors having low resistance values which are poorly matched to amplifiers of the read circuitry. In addition, parasitic resistances from layers of the read sensor that do not contribute to the magnetoresistance (e.g. the AFM layers) lower the signal-to-noise ratio (SNR) of the sensor. Finally, unlike magnetic tunneling junction (MTJ) CPP sensors using the current Ip as a tunneling current, the relatively low resistance of all metallic CPP sensors requires them to operate at very high current densities. However, effects such as the spin torque phenomenon and the Oersted field from the perpendicular current Ip limit current densities suitable for stable sensor operation.
Current densities of CPP read sensors may be increased by restricting the flow of the perpendicular current Ip through the sensor stack. Conventionally, this may be achieved by utilizing “current-screen” layers which are created by placing one or more ultra-thin insulating layers (a nano-oxide layer or NOL) within the sensor. Many tiny randomly-distributed conductive pores or holes, which restrict the current flow and concentrate the current density near the active layers of the sensor, are created through this process. In practice, however, the process is difficult to control and does not achieve adequate and manufacturable results. As sensors become smaller, the sensor covers such a small region of the film that statistical variations in the distribution of conductive pores, and therefore in the current density may cause uncontrollable and unacceptable variations in the sensor resistance.
Accordingly, there is an existing need to overcome these and other deficiencies of the prior art.